ANALYSIS OF GRAVITY AND MAGNETIC DATA IN THE BLUE MOUNTAINS PROVINCE, NORTHEASTERN OREGON

Rocks of the Blue Mountain province (BMP) of NE Oregon record a complex tectonic history of arc-related magmatism, basin formation, and terrane accretion. We seek to answer questions regarding the crustal structure produced by Mesozoic terrane accretion and younger tectonic and magmatic processes by performing a regional and local analysis of gravity and magnetic data synthesized with stratigraphic field studies. The analysis includes construction of isostatic gravity anomaly maps, low-pass filtered and horizontal derivative gravity and magnetic anomaly maps, and 2.5-D gravity and magnetic models. The BMP consists of early Mesozoic magmatic arcs (Wallowa and Olds Ferry terranes), a mélange belt (Baker terrane) and a large Triassic-Jurassic marine basin (Izee terrane) that were subject to Late Jurassic to Early Cretaceous deformation, metamorphism and emplacement 145-135 Ma plutons (Dorsey and LaMaskin, 2007; Brooks and Vallier, 1978). Mesozoic rocks are truncated on the east by the Salmon River Belt (SRB) and are locally covered by the Cenozoic Columbia River Basalt group. Our regional analysis highlights: (1) the Klamath-Blue Mtns lineament, a sharp NE-trending boundary between a massive gravity high in the NW and low values in the BMP, that truncates Mesozoic terrane boundaries and is truncated on the west by the Cascade volcanic arc; (2) the well-defined gravity high of the Snake River Plain; (3) gravity minima of the Wallowa, Bald Mtn, and Idaho batholiths; and (4) a deep gravity low east of Izee in the Izee basin. A more detailed study of the Izee area is being conducted using about 500 new gravity stations with an average station spacing of 1-3 km. The new gravity data will permit construction of a 2.5-D model that will more accurately delineate the extent and geometry of the Izee Basin and its geometrical relationship to adjacent geologic bodies. Low-pass filtered gravity and magnetic maps reveal a large, irregular gravity and magnetic minimum east of Izee that crosses the faulted boundary between the Baker and Izee terranes, possibly indicating a subsurface J-K or Tertiary batholith. Several smaller gravity lows, probably associated with felsic intrusions, overprint a weakly defined NE-trending low that appears to represent the subsurface Izee basin.